Low-power alternators of the abovementioned type, called magnetos, are well known. They are used, at the present time, on motor-assisted pedal cycles or on motor-cycles of small engine capacity, to provide the electrical supply. However, recent progress made in the technology of permanent magnets, bearings and surface treatments and in power electronics, makes it advantageous to use this form of alternator on motor vehicles. In fact, in relation to the conventional motor vehicle alternators, the alternators where induction is produced by a magnetic field created by permanent magnets have the advantage of not having any slip ring or brushes and of consequently benefiting from a reduced axial bulk. Like a conventional alternator, an alternator with permanent magnets is associated with subsidiary devices, particularly an electronic power switching device to rectify and to regulate the alternating current which is generated in the armature, and the output of such a switching device is connected to the battery of the vehicle and to all the consumer loads of the motor vehicle. Because of the quantity of heat released during operation, such an electronic rectifying and regulating device requires forced ventilation. Another subsidiary device for an alternator with permanent magnets which can be mentioned is that disclosed in French Patent Application No. 80.13645 filed on June 19, 1980. In this application, the rotor of the alternator is driven by means of a transmission having two gear ratios, the coupling being controlled as a function of the charge rate of the alternator or the speed of rotation of the internal combustion drive engine.